Evaluation of the solitary pulmonary nodule: size matters, but do not ignore the power of morphology

Subsequent to the widespread use of multidetector computed tomography and growing interest in lung cancer screening, small pulmonary nodules are more frequently detected. The differential diagnosis for a solitary pulmonary nodule is extremely broad and includes both benign and malignant causes. Recognition of early lung cancers is vital, since stage at diagnosis is crucial for prognosis. Estimation of the probability of malignancy is a challenging task, but crucial for follow-up and further work-up. In addition to the clinical setting and metabolic assessment, morphological assessment on thin-section computed tomography is essential. Size and growth are key factors in assessment of the malignant potential of a nodule. The likelihood of malignancy positively correlates with nodule diameter: as the diameter increases, so does the likelihood of malignancy. Although there is a considerable overlap in the features of benign and malignant nodules, the importance of morphology however should not be underestimated. Features that are associated with benignity include a perifissural location and triangular morphology, internal fat and benign calcifications. Malignancy is suspected in nodules presenting with spiculation, lobulation, pleural indentation, vascular convergence sign, associated cystic airspace, bubble-like lucencies, irregular air bronchogram, and subsolid morphology. Nodules often show different features and combination of findings is certainly more powerful. TEACHING POINTS • Size of a pulmonary nodule is important, but morphological assessment should not be underestimated. • Lung nodules should be evaluated on thin section CT, in both lung and mediastinal window setting. • Features associated with benignity include a triangular morphology, internal fat and calcifications. • Spiculation, pleural retraction and notch sign are highly suggestive of a malignant nature. • Complex features (e.g. bubble-like lucencies) are highly indicative of a malignant nature.